The generation of waste, particularly solid waste has become an increasingly worrisome environmental issue. Many landfills are becoming filled to the point where additional waste cannot be deposited therein. In addition, much of today's solid waste is not readily biodegradable, implying that the waste will not decompose in a timely manner. As an alternative, incinerators have been employed to burn solid waste, so as to minimize its physical footprint. However, these incinerators burn the waste and generate air pollutants that require very extensive gas cleanup, create ash that can be hazardous, and produce energy only in the form of heat, which is converted into electricity.
Plasma gasifiers offer an alternative to these current approaches. Plasma gasifiers use intense electrically based heating to enhance a gasification and melting process which produces a synthesis gas (syngas) consisting of hydrogen and carbon monoxide. Inorganic material is converted into a non-leachable glass. After cleaning, the synthesis gas can be preferably converted into a variety of liquid fuels or else combusted to produce electricity. Cleaning of the synthesis gas and recovering heat from the syngas can be a key part of the process.
FIG. 1 shows a representative plasma gasifier system. The plasma gasifier system 100 includes a reactor vessel 110, which is typically refractory lined. Within the vessel 110 are two or more electrodes 120a, 120b that are in electrical communication with one or more power supplies 130. In some embodiments, one electrode is suspended from the top of the reactor vessel 110, while the other electrode 120b is located at the bottom of the vessel. The power supplies create a significant electrical potential difference between the two or more electrodes, so as to create an arc. As waste is fed into the vessel 110 via a waste handler 140, it is exposed to extreme temperatures, which serve to separate the waste into its component parts.
The bottom of the vessel 110 contains molten metal 145. An area above the molten material forms an inorganic slag layer 147. Gasses, such as carbon monoxide and hydrogen gas, are separated and exit the vessel though portal 150. The gas, commonly known as syngas, exits the vessel 110 at an excessive temperature. Since the gas has not been processed, it is also referred to as dirty syngas. The syngas is cooled in a scrubber unit 180 to allow other particulates in the gas, such as carbon or sulfur to precipitate out of the gas. Halogens and acidic materials are removed from the syngas. The resulting gas is now referred to as clean syngas. The clean syngas can then be used to fuel a boiler or other device.
The plasma gasifier may also include joule heating of the molten material by passage of current between two or more electrodes that are immersed in the molten material 145.
In some embodiments, it may be advantageous to operate these plasma gasifiers at elevated pressure. While the throughput of the device is partially limited by the plasma power, it is possible to ease the requirements of the upstream/downstream gas handling equipment and the downstream catalyst by operating at elevated pressure of greater than one bar. For a given size, operating at increased pressure results in increased residence time, which is useful in achieving better mixing and increased conversion rates. Alternatively, the gas handling components of the system could be reduced in size, while maintaining a constant residence time, by operating at increased pressure. Operation at a slightly elevated pressure, such as 5 bar, is advantageous, as most of the advantages of higher pressure operation are obtained at this level, including a decrease in equipment size (such as pressure vessels and catalytic reactors used, for example in manufacturing liquid fuels). An optimum pressure range can be up to 10 bar, such as between 3 and 7 bar.
Operation at this higher pressure also helps regenerators used for heat recovery, due primarily to the reduced gas flow rates needed to exchange a given amount of energy.
Operation of a plasma gasifier at high pressure is inhibited by its adverse effect of the plasma characteristics. The high pressure operation of an arc plasma makes breakdown difficult and reduces the cross section of the arc plasma. For gasifier applications, it is disadvantageous that the plasma cross section decreases at elevated pressure with increased impedance. This decrease in size results in increased central temperatures, and increased interaction with the electrode materials. In addition, if the plasma is used to treat gas or liquids, there is reduced interaction with the environment due to the reduced cross sectional area. High pressure operation also results in plasma instability, where continuous plasma operation is difficult and the plasma extinguishes.
Therefore, there is a need for an effective apparatus and method to enable the advantages of high pressure operation, while overcoming the drawbacks listed above.